Hydraulic shock absorbers

All hydraulic shock absorbers work by the principle of converting kinetic energy (movement) into thermic energy (heat). For that purpose, fluid in the shock absorber is forced to flow through restricted outlets and valve systems, thus generating hydraulic resistance.
A telescopic shock absorber (damper) can be compressed and extended; the so called bump stroke and rebound stroke.

Telescopic shock absorbers can be subdivided in:
1. Bi-tube, or twin-tube dampers, available in hydraulic and gas-hydraulic configuration.
2. Mono-tube dampers, also called high pressure gas shocks.

Bi-Tube Shock Absorbers (fig. A and B)

The main components are:
- outer tube, also called reservoir tube (6)
- inner tube, also called cylinder (5)
- piston (2) connected to a piston rod (1)
- bottom valve, also called footvalve (7)
- piston rod guide (3)
- upper and lower attachment

For more information click on the picture:

How does a Bi-Tube shock absorber work?

Bump stroke
When the piston rod is pushed in, oil flows without resistance from below the piston through the orifices A, B, C and D and the non-return valve (19) to the enlarged volume above the piston. Simultaneously, a quantity of oil is displaced by the volume of the rod entering the cylinder. This volume of oil is forced to flow through the bottom valve into the reservoir tube (filled with air (1 bar) or nitrogen gas (4-8 bar). The resistance, encountered by the oil on passing through the footvalve, generates the bump damping.

Rebound stroke
When the piston rod is pulled out, the oil above the piston is pressurized and forced to flow through the piston. The resistance, encountered by the oil on passing through the piston, generates the rebound damping. Simultaneously, some oil flows back, without resistance, from the reservoir tube (6) through the footvalve to the lower part of the cylinder to compensate for the volume of the piston rod emerging from the cylinder.